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matt2491 02-15-2013 11:07 AM

12 bolt spider gears binding
Long story short, I need to replace the spider gears in my open diff '64 truck 12 bolt because I am converting from 17 spline to 30 spline axles. I bought a kit made by Superior that includes both side gears, spider gears, and thrust washers to go behind each gear. However, when I have it all installed in the carrier, the gears are virtually binding against each other. If I stick an axle in and try to turn the diff, it takes some effort and the gears are very notchy because there is zero play between them. Its like the thrust washers are too thick, especially the conical ones behind the spider gears. Without them in place however, the gears seem too loose.

I should note that the original spider gears did NOT have thrust washers behind them, but the side gears did.

Now I'm pretty confident this 12 bolt hadn't been abused. The whole truck is in good shape, and everything looked really good when I opened up the diff. The main ring & pinion backlash is perfect, the teeth on everything looked perfect, etc etc. So I don't think the carrier is warped or anything like that.

Anybody have any ideas about this? How are the spider gears supposed to feel? What kind of play is acceptable?

big gear head 02-15-2013 11:13 AM

Probably should have replaced the whole differential instead of just replacing the gears. GM usually makes some changes in the differential case when they change the axle splines or other parts. There are several versions of cases for the 7.5 and 8.5 rear ends because of minor changes that GM made when they changed axle splines. You can not run 28 spline gears in a 26 spline case in the 7.5 and you can not run 30 spline gears in a 28 spline case in the 8.5. You can not run 30 spline gears from the '90 to '98 case in the 30 spline '99 to current case in the 8.5. I would say that the inside dimintions of the case are not the same between the 17 spline version and the 30 spline version. Do not run it without the thrust washers. The gears will eat into the case.

rick 427 02-15-2013 11:20 AM

Thats good advice..and now that you have some knowledge now,I would still call Superior and talk with them about this,and hopefully be able to get your money back if it doesn't work out.

matt2491 02-15-2013 11:33 AM

Thanks guys for the advice!

I just called Superior and they said that it is good that the gears are tight. The guy said its better they be too tight than too loose. Does this sound right to you guys?

lmsport 02-15-2013 01:24 PM

There are different thickness shims for the axle side gears.
I know I have a big box of them, so try to find a shop that has some.

painted jester 02-15-2013 02:51 PM

Tights good, loose is bad !! You installed new spiders & shims they need to be broke in, drive till hot, let cool , drive till hot let cool again, Do this several times and the gears will break in with a good pattern!


prostreet6t9 02-16-2013 08:52 PM


Originally Posted by lmsport (Post 1646810)
There are different thickness shims for the axle side gears.
I know I have a big box of them, so try to find a shop that has some.

The Aftermarket Side & Spider kits Don't come with Selectable thrust washers.

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